void reverse(char *s) {
  char c, *end;
  end = s + strlen(s) – 1;
  while (s < end) {
    // 2 ends approach center
    c = *s;
    *s++ = *end;
    // or *s = *end; s++;
    *end‐‐ = c;
    // or *end = c; end‐‐;
  }
}